Magnetically dead layer in interacting ultrafine NiFe2O4 nanoparticles

Abstract

The interplay of the magnetically dead layer and structural defects in interacting ultrafine nickel ferrite (NiFe₂O₄) nanoparticles (<d> = 4 nm) have been investigated using transmission electron microscopy, X-ray diffraction, ⁵⁷Fe Mössbauer spectrometry, and static (dc) magnetization and dynamic (ac) susceptibility measurements. According to the magnetic measurement data, there are three magnetic subsystems in NiFe₂O₄ nanoparticles. The first subsystem with the lowest blocking (spin freezing) temperature (T_S = 8 K) involves atomic magnetic moments of magnetically disordered particles with a size of d < 4 nm. The other two subsystems are formed by magnetic moments of the cores of nanoparticles more than 4 nm in size and by correlated surface spins in nanoparticle clusters. The magnetic moments of the ferrimagnetically ordered cores are blocked at a higher temperature (∼40 K). It has been shown that the most significant contribution to the energy dissipation is made upon blocking of the correlated nanoparticle surface spins from the magnetically dead layer on the nanoparticle surface. The magnetic measurements have shown that the thickness of this layer is d_md ≈ 1 nm for a particle with a diameter of < d> = 4 nm. At the same time, the ⁵⁷Fe Mössbauer spectrometry study has revealed a structural disorder penetrating to a depth of up to d_cd ≈ 0.6 nm in a particle with a diameter of < d> = 4 nm. This evidence for a faster violation of the magnetic order than in the case of the crystal order upon moving away from the center of a particle to its periphery.